Single plane typically has better air fuel distribution and more power output capability. It also usually brings the powerband up a little compared to it's dual plane counterpart, but not always and not necessarily by much.
Truly, I've run three intakes on my mopars. The Indy dual plane, which is a pile of ****. The eddy RPM intake, which is wonderful and makes great power across a wide RPM range and the eddy victor, which also performs great across a wide powerband. The victor can cause some clearance issues if you don't have a scoop.
To answer your original questions.
1. Depends on the application and the intake.
2. Depends on the application and the engine build.
